When a food has, hitherto, been cooked with heat in a home, the food was wrapped up in a wrapping film for home use or was packed in an air or moisture impermeable packaging bag and was sealed, and it was heated in a microwave oven. In such a case, moisture contained in the food was evaporated and thus, was burst the bag by an internal pressure. Even if the bag was not burst, the food became sticky due to dew condensation, and its taste was impaired. Then, there is disclosed a cooking method wherein a food is packed in, as a food packaging bag utilized in cooking using a microwave oven, a two-layer packaging bag comprising an inner packaging material composed of an air permeable material such as a perforated film or a non-woven fabric and an outer packaging material composed of an air impermeable film which acts so as not to enter dust into the inside, as proposed in Japanese Examined Utility Model Publication No. 63-18607, and when cooking it in a microwave oven, after a part encompassed by a perforation, which is provided on a part of the outer packaging material, is removed, and it is cooked.
On the other hand, in Japanese Unexamined Patent Publication No. 58-116666 and No. 58-134980, there is disclosed a preparation method of a packed food wherein a food is packed in a finely porous material on which fine holes are innumerably perforated, provided for escaping steam generated on heat-cooking out of the bag to prevent an increase of the internal pressure and for preventing an invasion of microorganisms from the outside, and then is subjected to sterilization with heat treatment.
Also, in Japanese Unexamined Utility Model Publication No. 59-117382, it is disclosed to use a finely porous polyethylene film which is permeable to steam but is impermeable to water as a packaging material for food to be steam-cooked.
Also, in Japanese Unexamined Patent Publication No. 63-92316, there is disclosed a sheet or bag for cooking comprising a water-repellent and porous sheet having steam permeability.
Also, in Japanese Unexamined Utility Model Publication No. 62-191675, there is described a method wherein a water-absorbent sheet previously containing a necessary amount of water is provided at a bottom part of a packaging bag, and steam is generated in the inside of the packaging bag on heat-cooking, thereby preventing to dry a food, as a result, the steam-cooking can be conducted.
The packaging material of Japanese Examined Utility Model Publication No. 63-18607 has, however, a superposition structure of an inner layer composed of a perforated material which is incompletely waterproof such as a synthetic film having a large number of circular small holes, a synthetic resin non-woven fabric or synthetic resin net, and an outer layer composed of a film having a part encompassed by a perforation, which is to be removed. On cooking, it is necessary that a part of the outer layer film is cut and removed, which is troublesome. Also, since the perforated material of the inner layer is bonded to the outer layer which is impermeable to moisture excepting for the part to be cut and removed, steam generated on heat-cooking cannot go out into the outside. That is, the effect obtained by using the perforated material as the inner layer can insufficiently be exhibited. Furthermore, there is a case where steam passing through the perforated material of the inner layer is condensed to produce water, and the water reverses through the perforated material part which is incompletely waterproof into the inside again, so the effect of dissolving that the food in the bag becomes sticky can be insufficiently exhibited.
On the other hand, the bags using, as the moisture permeable layer, the finely porous film or sheet which is permeable to steam but is impermeable to water, and is water-repellent alone, as discussed in Japanese Unexamined Patent Publication No. 58-116666 and No. 58-134980, Japanese Unexamined Utility Model Publication No. 59-117382, and Japanese Unexamined Patent Publication No. 63-92316, are easily broken during distribution and heat-cooking, that is, they are practically unsatisfactory as the commodities. On the other hand, as a solving means of the problem that the strength is weak, it can be considered that the strength reinforcement can be improved by putting another air permeable material which is excellent in strength such as a non-woven fabric, a woven fabric, a perforated film, a cross-overlaying sheet or a net over the film or sheet on the whole, using an adhesive or by heat-sealing. However, according to such a means, the functions existing in the finely porous film such as moisture permeability are impaired, and there is a defect that the cost of the packaging material is increased because the step for laminating is increased.
Also, the packaging bag wherein the water-absorbent sheet previously containing water is provided on the bag, as discussed in Japanese Unexamined Utility Model Publication No. 62-191675, has no particular hindrance during storage and transportation in case of packaging a frozen food, because water is frozen. However, in case of storing or transporting a non-frozen food, the bag gives a problem that the food of the content is wet with water contained in the sheet. Also, if the food contains originally water, it is not required to contain water in the sheet.